Blank Canvas: Add the "Subscription Form and Links" pattern

[kjellr]

Adds the Jetpack Subscribe + Links pattern from #3161, now that Automattic/jetpack#18085 has been resolved. This PR also properly escapes the wpcom-only block pattern strings + URLs to match up with the changes made in #3345.

Two small caveats:

• The design is changed slightly from the mockup in the other thread: I've removed the noisy background so that the form field text is more readable.
• It's pretty obvious that the two 50% buttons don't add up to the full width of the container. This is a Gutenberg bug, and there's a potential fix here. I don't think that should hold this PR up though.

Screenshots

Editor:

Front End:

Desktop	Mobile

[jffng]

I got a block validation error when I added this pattern:

Clicking "Attempting Block Recovery" worked though.

[scruffian]

LGTM. I didn't see the validation error. The jetpack form looks very different on the front/back but that's outside of the scope of this...

[kjellr]

@jffng were you testing on WP.com? I just want to make sure we can't reproduce before merge.

[jffng]

Yes — tested on a simple site. I will give it another spin on different site to see if I can still recreate it.

[jffng]

Confirm this happens every time I insert the pattern on a sandboxed simple site. This is the console error:

index.js:13 Block validation: Block validation failed for `jetpack/subscriptions` (Object).

Content generated by `save` function:

<div class="wp-block-jetpack-subscriptions wp-block-jetpack-subscriptions__supports-newline">
			[jetpack_subscription_form
				subscribe_placeholder="Enter your email address"
				show_subscribers_total="false"
				button_on_newline="false"
				submit_button_text="Sign Up"
				custom_background_emailfield_color="undefined"
				custom_background_button_color="undefined"
				custom_text_button_color="undefined"
				custom_font_size="16"
				custom_border_radius="0"
				custom_border_weight="1"
				custom_border_color="undefined"
				custom_padding="16"
				custom_spacing="10"
				submit_button_classes="has-text-color has-primary-color has-background has-background-background-color"
				email_field_classes=""
				show_only_email_and_button="true"
			]</div>

Content retrieved from post body:

<div class="wp-block-jetpack-subscriptions wp-block-jetpack-subscriptions__supports-newline">[jetpack_subscription_form subscribe_placeholder="Enter your email address" show_subscribers_total="false" button_on_newline="false" custom_font_size="undefinedpx" custom_border_radius="0" custom_border_weight="1" custom_padding="16" custom_spacing="10" submit_button_classes="has-undefinedpx-font-size has-text-color has-primary-color has-background has-background-background-color" email_field_classes="has-undefinedpx-font-size" show_only_email_and_button="true"]</div>

[kjellr]

@jffng I tried on a couple test sites and was unable to reproduce. Is your sandbox up to date?

[jffng]

It was not! Working now 🚢